Deflecting circuit for cathode ray tubes



Jan. 27, 1942. A. R. GRENFELL DEFLECTING CIRCUIT FOR CATHODE RAY TUBES Filed June 4, 194

A 7 mm 6 ww 6 MM m Flu/n m T. A lsST I II R .SAw Toom Vow/w: Sou/res INVENTOR A.R. GRENFELL A TTORNE Y Patented Jan. 27, 1942 DEFLECTING CIRCUIT FOR. CATHODE RAY TUBES Alexis Ren Grenfell, Stanmore, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application June 4,

1 Claim.

This invention relates to electrical circuits for generating oscillatory currents or voltages of saw-tooth waveform as applied to the deflecting coils or plates respectively of a cathode ray tube for deflecting the electron beam.

In certain forms of cathode ray tube, the electron beam is required to scan a rectangular area on a screen disposed at an angle to the axis of the beam. Thus, the tubeused in the transmitting camera in a television system may have a picture resolving screen of the mosaic type, the image to be transmitted being formed upon the same side of the screen as that upon which the electron beam impinges. If the deflection is effected in the normal manner using electrical oscillations of rectilinear saw-tooth form and constant amplitude for the deflection in both coordinates, the area scanned upon the screen will be substantially of keystone shape.

In the specification of United States Patent No. 2,176,973, a method of compensating for the effect referred to is described. In that specification it is proposed to modulate with potentials proportional to the frame deflecting waveform the valve controlling the charging of a condenser across which the line frequency deflecting waveform is set up. The effect is that the amplitude of the line deflecting waveform at which discharge of the condenser occurs is varied. The potential at the end of the discharge remains at a fixed value and it is therefore necessary to add into the waveform, a component proportional to the frame deflecting waveform in the correct sense to restore symmetry of the scanned area. Such addition may be effected in a subsequent amplifying stage but the use of an extra valve is involved in order to effect mixing.

The object of the present invention is to provide an improved circuit for generating deflecting fields which will effect scanning over a rectangular shaped area on an inclined screen or a keystone shaped area over a screen at right angles to the axis of the electron gun.

According to the present invention, in a circuit arrangement for the production of a scanning fleld by deflecting an electron beam in two mutually perpendicular directions and employing substantially saw-tooth waveforms of different frequencies, the long flanks of the saw-tooth waveforms of higher frequency are generated across a storage device and the short flanks of said waveforms are generated by switching into operation a thermionic valve which is arranged to charge rapidly the energy stored in said device and the saw-tooth waveforms of higher fre- 1940, Serial No. 338,700 In Great Britain June 6, 1939 quency are controlled in amplitude by the sawtooth waveforms of lower frequency which are applied to a control electrode of a further valve, an output electrode of which is connected to said storage device so as to control the change of energy in said device, said further valve also providing an output which serves to control the cathode potential of said first valve in such a manner that when it is switched into operation to generate the short flanks of the higher frequency waveforms, the change of energy in said storage device is controlled in such a manner as to produce symmetry in the scanning field produced.

In one form of circuit arrangement embodying the invention an impedance is connected in the cathode lead of said further valve, said impedance being of such a nature that the potential of the cathode of said further valve follows the potential variations of its control grid, and the cathode of said first mentioned valve is so connected to the cathode of said further valve that when it is switched into operation to generate the short flanks of the higher frequency waves the change of energy in said storage device is controlled by the cathode potentlal of said further valve. In the arrangement referred to the anodes of the valves are connected together by the anode resistance of one of the valves.

In a more detailed circuit for generating line frequency deflecting waveforms for television purposes, a valve fed with pulses at the line frequency and controlling the charge in a condenser across which the line deflecting waveform is set up has its anode connected through a resistance to the anode of a second valve or to a point in a resistance in the anode lead of that valve, the grid of which is connected to a source of frame frequency saw-tooth scanning potential. The cathode of the first valve is connected to the cathode of the second valve which has a resistance in its cathode lead, or to a point in that resistance.

In order that the invention may be clearly understood and readily carried into efiect a circuit arrangement embodying the invention will now be described by way of example with reference to the accompanying drawing.

Referring to the drawing, the valve V1 constitutes the usual valve controlling the charge in a condenser C across which the line deflecting saw-tooth waveform is set up. The control grid of the valve V1 is connected to a source I of line frequency pulses which are applied to the control grid in a positive sense. The condenser C is charged through resistancesRl and R: from a source of potential not shown, the charging voltage being modified by variations in the voltage at the anode of a second valve Va, the control grid of which is connected to a source 2 of frame frequency scanning saw-tooth. potentials.

The cathode of the valve V1 is connected to an adjustable tapping point in the resistance R3 connected in series with a resistance R4 in the oathode lead of the valve V2. The connection of the cathode of the valve V1 in the manner indicated ensures that the voltage to which the condenser C is discharged at the end of each line shall vary in the same way but in opposite phase to the' modified by the varying potential at the anode of the valve V2 and the degree of the charge acquired by the condenser C will be different for each line period. Similarly, as the potential at the cathode of the valve Vi follows that of the cathode of the valve V2, which in turn varies in opposite phase with that at the anode, the voltage to which the condenser C is discharged at the end of each line will be controlled by the cathode potential. The amplitude of the line saw-tooth deflections will thus decrease symmetrically and a symmetrical keystone shaped scanning patch will be produced.

The ratio between the resistance R5, which is constituted by the resistance 12.4 added to that portion of the resistance R3 below the tapping point connected to the cathode of the valve V1 and the resistance R1 is of importance since a larger modulating voltage must necessarily be present across the resistance R1 than across the resistance R5 owing to the division of potential due to the resistance R2 and the condenser C.

It can be shown approximately that where t is the time that elapses between successive line pulses arriving at the grid of the valve Vi. The potentiometer connection in the cathode lead of the valve V2 serves to provide fine adjustment for symmetry of the scanned patch produced and the total amount of modulation by the frame frequency saw-tooth potentials can be varied by controlling the input to the grid of the valve V2. Conveniently, the amount of such modulation may be a predetermined fixed proportion of the frame saw-tooth deflecting waveform used for frame scanning in the cathode ray tube connected to the circuit. By this means a1- t'eration of the height of the scanned patch will give the necessary alteration to the amount of keystone modulation to retain a rectangular scanned patch.

Variations may, of course, be made in the form of the circuit shown in the drawing. Thus, for example, while the anode of valve V1 is shown connected directly through resistance R2 to the anode of valve V2, it may be connected via resistance R: to a pre-selected point in the resistance R1 or to an adjustable tapping point in that resistance. Again, the cathode of the valve V1 may be connected directly to the cathode of the valve V2 or to a rare-selected fixed point or adjustable tapping point in the resistance R3, or again it may be connected between the two resistances R3 and Ba. Further, the resistances R3 and R4 may be replaced by a single resistance.

It will be understood that while in the example described the potential controlling the voltage to which the condenser C is discharged has been taken from the cathode of the valve V2, the appropriate output may be derived in alternative ways. Thus in the case of a multi-grid valve, the control potential may be taken from any electrode in the valve the potential of which varies in the appropriate sense as, for example, the screening grid in a pentode or a hexode valve.

I claim:

An apparatus of the kind described comprising an electron tube having a cathode, a control electrode and an anode, means including a resistance for connecting the cathode of said electron tube to a point of fixed potential, means including a load resistance for maintaining the anode positive with respect to the cathode, means for applying voltage variations of saw-tooth wave form and of a predetermined frequency between the control electrode and the cathode of said electron tube, an electron discharge device comprising a cathode, a control electrode and an anode, means for connecting the cathode of said electron discharge device to a point along said first named resistance, means including a further resistance for connecting the anode of said discharge tube to the anode of said electron tube, a condenser connected between the anode of said discharge tube and said point of fixed potential, and means for applying control impulses to the control electrode of said electron discharge device, the frequency of said control impulses being large as compared to the frequency of the voltage variations of saw-tooth wave form, whereby amplitude modulated voltage variations of saw-tooth wave form may be developed across said condenser.

ALEXIS RENE GRENFELL. 

